1. Field of the Invention
The present invention relates to communications, and, in particular, to cellular telecommunication systems.
2. Description of the Related Art
A typical cellular telecommunication system has one or more mobile switching centers (MSCs), for example, located in the central offices of a company that provides telecommunication services, where each MSC controls the operations of multiple cell sites. A cell site (also referred to as a cell) corresponds to a set of co-located uplink and downlink antennas that support communications with the mobile units (e.g., cellular telephones) currently located within a coverage area around the cell site. The locations of cell sites are preferably distributed to provide contiguous coverage over a relatively wide telecommunication service region. In order to ensure this contiguous coverage, cell sites are positioned such that their coverage areas overlap to some extent with the coverage areas of adjacent cell sites. This overlapping also enables a mobile user to move from one cell site to another without losing telecommunication service. This process is referred to as a handoff, as the responsibility for communicating with the mobile unit is handed off (i.e., transferred) from one cell site to the other.
One type of cellular system uses both frequency division multiple access (FDMA) and time division multiple access (TDMA) communication techniques. Such a cellular system is typically assigned two different frequency bands for its communications: one for downlink communications from the cell sites to the mobile units and the other for uplink communications from the mobile units to the cell sites. Pursuant to FDMA schemes, each frequency band is divided into multiple frequency sub-bands called channels. Pursuant to TDMA schemes, communications on each channel are time-multiplexed into different periodic time slots. When communications are to be established between a cell site and a mobile unit located within the coverage area of the cell site, the mobile unit is assigned a particular channel and time slot in each of the downlink and uplink frequency bands within which signals to and from the mobile unit are to be transmitted. Responsibility for assigning channels and time slots rests with the MSC, which is constantly receiving information from and transmitting instructions to its cell sites.
One of the primary functions of the MSC is to assign channels and time slots so as to minimize interference between the mobile units currently in use within the overall telecommunication service region. If two mobile units located within the same cell site or even within adjacent cell sites are assigned the same channel and time slot, their signals will interfere with one another which may result in corrupted communications. Such interference may even result when the mobile units are separated from one another by one or more intervening cell sites. The MSC is responsible for assigning channels and time slots to all of the mobile units in an efficient and effective manner with minimization of interference, while achieving high capacity as a goal.
One traditional scheme for assigning channels is referred to as fixed channel assignment (FCA) in which specific channels are manually and permanently assigned to specific cell sites. Under FCA, channels are assigned when the cellular system is designed such that the same channels are not available in adjacent and perhaps even in relatively nearby cell sites. In this way, it can in theory be guaranteed that no two mobile units will interfere with one another. FCA has a number of limitations. First of all, when new cell sites are added to the overall coverage region, the task of assigning channels to the new cell site and possibly reassigning channels to the existing cell sites can be extremely complicated, especially when a new cell site is added to the interior of the overall system coverage region.
In addition, permanently assigning channels to cell sites does not take into consideration the fact that the number of mobile users within a cell site will vary over time. For example, if a football stadium and the surrounding parking lots are covered by different cell sites, the number of mobile units that need to be supported within the football stadium during a football game will be much greater than the number of mobile units in the parking lots. By the same token, the opposite will be true immediately before and after the game. As illustrated by this example, permanently assigning channels to cell sites leads to inefficient allocation of communication resources.
Another scheme for assigning channels is referred to as dynamic channel assignment (DCA) in which channels are assigned to cell sites in real time as they are needed to support communications with mobile units within the coverage areas of the different cell sites. DCA offers better performance than FCA due to more efficient trunking (i.e., assignment) of channels. Higher efficiency is obtained since all channels in the spectrum are potentially available for use at each cell (or sector for those cells that are divided into multiple sectors). DCA schemes fall into two classes. The first is based on radio frequency (RF) measurements; the second is based on utilizing database information to enforce a minimum reuse-distance criterion. In real-world cellular systems, an algorithm belonging to one class or the other has inherent advantages and disadvantages due to the characteristics of the information used to make decisions about channel assignment.